Formation of amorphous sugar in the syrup film – a key factor in modelling of industrial sugar drying

A mathematical model of the steady-state sugar drying operation in a cross-flow rotating drum dryer is proposed. Although the model is based on the classic two-film concept of simultaneous heat and mass transfer, it also allows for the formation and growth of a thin layer of amorphous sugar due to local supersaturation of the sugar syrup. The formation of amorphous sugar is a result of the competition between crystallization and drying kinetics. The onset of amorphization is assumed to occur when the system moves from the metastable zone to the labile zone on the sucrose–water phase diagram. The rate of sucrose amorphization is controlled by the conditions prevailing on the metastability limit. Diffusivity of water through the amorphous sugar, the only adjustable parameter of the proposed model, has been determined by fitting sugar moisture data from an industrial Louvre-type dryer. The layer of amorphous sugar plays a critical role during the falling-rate period of drying. Generally, models ignoring its presence tend to overestimate the moisture removal rate. The presented model predicts a dramatic decline of the evaporation rate once the amorphous sugar begins to form which is observed in the industrial practice. The effect of key operating parameters such as air flow rate, hot air temperature, and crystal size on the dryer performance is also discussed.